One conventional method for measuring the hardness of a tissue material of a living body is a method in which a probe is pressed against the material to be measured, vibration is applied thereto, mechanical vibration response of the tissue material to the input vibration is detected using a sensor, and characteristic values corresponding to the hardness are obtained based on variations in the frequency, phase, etc. Examples of this method are disclosed in, for example, “Measurements of the Hardness of a Soft Material with a Piezoelectric Vibrometer and Their Analysis” (Sadao Omata, Medical Electronics and Bioengineering, Vol. 28, No. 1, 1990, pp1-8) and “New tactile sensor like the human hand and its applications” (S. Omata et al, Sensors and Actuators A, 35 (1992) pp9-15). FIG. 7 shows an apparatus for measuring the hardness of tissue of a living body disclosed in Japanese Patent Laid-Open Publication No. Hei 9-145691. FIG. 7 shows a probe unit 2 pressed against the material 1 to be measured of a living body, for example, the tissue of the skin or the tissue of the viscus, such as a liver tissue exposed when the abdomen is opened. The probe unit 2 includes a vibrator 3 and a strain detection sensor 4, and is connected to an external control unit 5. The control unit 5 has a self-excited oscillation circuit 6 and the self-excited oscillation circuit 6 has an amplifier 7 and a gain variation correction circuit 8. Furthermore, the self-excited oscillation circuit 6 is connected to a frequency measuring circuit 9 to measure its frequency and a voltage measuring circuit 10 to measure its amplitude.
The operation of this conventional example will be explained below. When the probe unit 2 is pressed against the material 1, the vibrator 3 inside the probe unit 2 generates self-excited vibration as an electric signal is converted to mechanical vibration by a mechanical/electrical vibration system of the vibrator 3 and the self-excited oscillation circuit 6 in the control unit 5, and the vibration is input from the end of the probe unit 2 to the material 1. The material 1 responds to this input vibration according to its mechanical vibration transmission characteristic. The strain detection sensor 4 detects this output vibration (strain) and converts it to an electric signal. The vibrator 3 and the strain detection sensor 4 can be implemented by, for example, a piezoelectric vibration element and a piezoelectric sensor. The vibrator 3 and the strain detection sensor 4 together with the amplifier 7 form a feedback loop and then oscillation is self-excited. Here, as a result of the material 1 responding to the input signal from the vibrator 3, generally the frequency changes, a phase difference is generated and the amplitude is reduced. But the gain variation correction circuit 8 has a function of correcting the amplitude gain of the output signal of the strain detection sensor 4. Furthermore, because the gain variation correction circuit 8 is formed in the feedback loop of the self-excited oscillation circuit 6, feedback is provided in such a way that the phase difference generated becomes to zero while the amplitude gain is being corrected. When the phase difference is fed back to zero, the resonance frequency of the mechanical/electrical vibration system, which includes the material 1, the vibrator 3, the self-excited oscillation circuit 6, and the strain detection sensor 4, can be obtained by the frequency measuring circuit 9.
As is well known, because this resonance state is a speed resonance state, the resonance amplitude reaches its maximum value when the phase is zero, irrespective of the damping constant of the system; this is unlike a displacement resonance state or an acceleration resonance state. Therefore, as irrespective of the damping constant, it is possible to calculate a spring constant of the material by obtaining the resonance frequency when the phase is zero. Therefore, a frequency variation df between this resonance frequency and the frequency when the probe unit 2 is not in contact with the material 1 is the characteristic value corresponding to the hardness of the material 1. For example, FIG. 8 shows a relationship between the frequency variation df and the shear modulus G measured by another method in the case of a gelatin of 30 mm in thickness. FIG. 9 shows values of frequency variation df of various materials containing tissues of a living body using foam rubber as a reference. From the frequency variation df measured in this manner, it is possible to calculate the hardness of the material 1 based on the correlation with the shear modulus G, the correlation with the Young's modulus using a known relational expression and the correlation with spring constant of the material 1 against which a specimen having a certain diameter is pressed.
The apparatus for measuring the hardness of a material in the above-described conventional example uses a response of the material 1 which is excited by the vibrator 3, that is the response of the vibration transmission characteristic, including the spring constant of the material 1, under the speed resonance state. In this manner it is possible to obtain a numerical value for the characteristic value corresponding to the hardness of tissue of a certain living body which is the material 1. However, this configuration requires that the probe unit 2 be directly pressed against the material 1 to provide a signal from the vibrator 3 to the material 1. In other words, to obtain a characteristic value corresponding to the hardness of a tissue material inside a living body, it is necessary to dissect or open the abdomen of the living body, remove other tissue materials of the living body that block access to the material 1, and then directly press the probe unit 2 against the material 1.
The present invention provides a method and an apparatus for measuring material characteristics capable of solving the problems of the above-described conventional technology and easily measuring characteristic values corresponding to the hardness of the material to be measured inside the living body using the response of a vibration transmission characteristic without the need to remove other tissue materials of the living body by dissecting or opening the abdomen of the living body.